selftests: net: add a hint about MACAddressPolicy=persistent

selftests: net: add a hint about MACAddressPolicy=persistent

New NIPA installation had been reporting a few flaky tests.
arp_ndisc_evict_nocarrier is most flaky of them all.
I suspect that the flakiness is due to udev swapping the MAC
addresses on the interfaces. Extend the message in
arp_ndisc_evict_nocarrier to hint at this potential issue.
Having the neigh get fail right after ping is rather unusual,
unless udev changes the MAC addr causing a flush in the meantime.

Link: https://patch.msgid.link/20251127194556.2409574-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>